Grain-based spirits can be distilled either after removing the grain solids, or with the grain solids remaining in the “mash,” which is called “grain-in” distilling. Grain-in batch distilling has several advantages, including more efficient conversion of the grain to alcohol and, in the case of some spirits, a better tasting product.
Batch stills are available that employ one of four types of heating systems: direct flame heating; immersion electric heating elements; liquid-jacketed heating and steam-jacketed heating. Direct flame and immersion electric heating element stills are not practical for grain-in distilling. Their localized, high-temperature heating surfaces scorch the mash when operated at higher levels. As a result, the heat settings in these stills must be turned down to such an extent during a grain-in batch run that the distillation time becomes impractically long.
Liquid-jacketed stills circulate an externally heated liquid through a cavity formed by a double-wall of the still. They are capable of grain-in distilling without scorching because the heat is distributed over a large surface area with the result that no hot spots are produced. However, liquid jacketed stills are very slow to reach distilling temperatures, and they are very slow to respond when the operator changes the heat settings.
Steam-jacketed stills employ an external steam boiler and the steam is pumped through pipes to a cavity formed by a pressure-tested double wall of the still. These stills are the mainstay of the commercial distilling industry. They heat quickly, are responsive to changes in heat settings, and can in a timely manner distill grain-in without scorching . However, the pressure-tested double-wall still and the steam boiler system required to power it, are very expensive.